The identification of objects, such as packages, retail sales items, shipping containers, and the like, is often accomplished by affixing an information storage medium such as a `bar code` label to an object. The bar code is encoded with previously assembled and stored information that fully describes the object. In order to read the label, the object must be oriented so that the bar code may be `viewed` by an optical (e.g. laser) scanner, either as a stationary device or a hand held unit.
A familiar example of the use of such a scanner is a check-out line of a retail sales establishment, where a clerk orients each item with the bar code label face down, and then passes the item across a generally horizontal viewing window. As the object is passed over the window, the bar code is read by the scanner located beneath the window, which outputs data to an adjacent point-of-sales terminal, where the sales transaction is processed. If the object is not properly oriented to allow the scanner to read the label, the sales clerk will not hear an audible tone confirming success of the scan, and will repeat the process.
A similar technique is employed in the transportation industry, where items (e.g., baggage) being unloaded from a vehicle (e.g, aircraft) are placed upon a conveyor belt in a prescribed orientation, so that they may be viewed by the scanner. If the object has not been properly placed on the conveyor, the object will require further handling by other personnel to properly orient the bar code label so that it can be read by a downstream (stationary or hand held) scanner. This need to physically orient encoded label-containing items relative to an optical scanner is both labor-intensive and time-consuming, and constitutes an unwanted expense.
Non-limiting examples of proposals to address this problem include a variety of transponder tag-based systems, such as those described in the Bickley et al, U.S. Pat. No. 5,430,441; Watanabe et al, U.S. Pat. No. 5,478,991; and Brooks et al, U.S. Pat. No. 5,485,154. In each of these schemes, the tag is powered by an exciting source that continuously generates an exciting magnetic field to power the tag. In the Bickley et al and Watanabe et al approaches the tag responds by modulating the continuously transmitted field. In the Brooks et al scheme, the tag responds by generating a signal at a frequency different from the exciting frequency to avoid interference. An obvious drawback of each patented approach is the fact that they continuously consume power. Indeed, in the Brooks et al scheme, the amount of excitation energy required is so large, that it requires the use of a shielded tunnel to prevent electromagnetic contamination of the surrounding environment.